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<div id="app"><!--[--><!--[--><!--[--><span tabindex="-1"></span><a href="#main-content" class="skip-link sr-only">Skip to content</a><!--]--><div class="theme-container has-toc"><!--[--><!--[--><header class="navbar"><div class="navbar-left"><button class="toggle-sidebar-button" title="Toggle Sidebar"><span class="icon"></span></button><!----><a href="/" class="brand"><img class="logo" src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/logo-02.png" alt="Java 程序员进阶之路"><!----><span class="site-name hide-in-pad">Java 程序员进阶之路</span></a><!----></div><div class="navbar-center"><!----><nav class="nav-links"><div class="nav-item hide-in-mobile"><a href="/blog.html" class="nav-link" aria-label="博客"><span class="icon iconfont icon-gaishu" style=""></span>博客<!----></a></div><div class="nav-item hide-in-mobile"><a href="/home.html" class="nav-link" aria-label="进阶之路"><span class="icon iconfont icon-lujing" style=""></span>进阶之路<!----></a></div><div class="nav-item hide-in-mobile"><a 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class="button-bottom"></span></span></button></div></header><!----><!--]--><!----><div class="toggle-sidebar-wrapper"><span class="arrow left"></span></div><aside class="sidebar"><!--[--><!----><!--]--><ul class="sidebar-links"><li><!--[--><a href="/home.html" class="nav-link sidebar-link sidebar-page" aria-label="一、前言"><!---->一、前言<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">二、Java核心</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.1 Java概述</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.2 Java语法基础</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.3 数组&字符串</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.4 面向对象编程</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.5 集合框架（容器）</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 IO</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.7 异常处理</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">2.11 并发编程</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/thread/wangzhe-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="创建Java线程的3种方式"><!---->创建Java线程的3种方式<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-state-and-method.html" class="nav-link sidebar-link sidebar-page" aria-label="线程的6种状态及切换"><!---->线程的6种状态及切换<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-group-and-thread-priority.html" class="nav-link sidebar-link sidebar-page" aria-label="线程组和线程优先级"><!---->线程组和线程优先级<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/why-need-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="进程与线程的区别"><!---->进程与线程的区别<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-bring-some-problem.html" class="nav-link sidebar-link sidebar-page" aria-label="并发编程带来了哪些问题"><!---->并发编程带来了哪些问题<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/jmm.html" class="nav-link sidebar-link sidebar-page" aria-label="Java内存模型"><!---->Java内存模型<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/volatile.html" class="nav-link sidebar-link sidebar-page" aria-label="volatile"><!---->volatile<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/synchronized.html" class="nav-link sidebar-link sidebar-page" aria-label="synchronized"><!---->synchronized<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/cas.html" class="nav-link sidebar-link sidebar-page" aria-label="CAS的原理"><!---->CAS的原理<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/aqs.html" class="nav-link sidebar-link sidebar-page" aria-label="AQS详解"><!---->AQS详解<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/thread/lock.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="锁"><!---->锁<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#synchronized的不足之处" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="synchronized的不足之处"><!---->synchronized的不足之处<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#锁的几种分类" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="锁的几种分类"><!---->锁的几种分类<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#可重入锁和非可重入锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="可重入锁和非可重入锁"><!---->可重入锁和非可重入锁<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#公平锁与非公平锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="公平锁与非公平锁"><!---->公平锁与非公平锁<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#读写锁和排它锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="读写锁和排它锁"><!---->读写锁和排它锁<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#jdk中有关锁的一些接口和类" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="JDK中有关锁的一些接口和类"><!---->JDK中有关锁的一些接口和类<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#抽象类aqs-aqls-aos" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="抽象类AQS/AQLS/AOS"><!---->抽象类AQS/AQLS/AOS<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#接口condition-lock-readwritelock" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="接口Condition/Lock/ReadWriteLock"><!---->接口Condition/Lock/ReadWriteLock<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#reentrantlock" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ReentrantLock"><!---->ReentrantLock<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#reentrantreadwritelock" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ReentrantReadWriteLock"><!---->ReentrantReadWriteLock<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/lock.html#stampedlock" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="StampedLock"><!---->StampedLock<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li></ul><!--]--></li><li><!--[--><a href="/thread/reentrantLock.html" class="nav-link sidebar-link sidebar-page" aria-label="重入锁ReentrantLock"><!---->重入锁ReentrantLock<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ReentrantReadWriteLock.html" class="nav-link sidebar-link sidebar-page" aria-label="读写锁ReentrantReadWriteLock"><!---->读写锁ReentrantReadWriteLock<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/condition.html" class="nav-link sidebar-link sidebar-page" aria-label="线程协作类Condition"><!---->线程协作类Condition<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/LockSupport.html" class="nav-link sidebar-link sidebar-page" aria-label="线程阻塞唤醒类LockSupport"><!---->线程阻塞唤醒类LockSupport<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/map.html" class="nav-link sidebar-link sidebar-page" aria-label="并发集合容器"><!---->并发集合容器<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ConcurrentHashMap.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentHashMap"><!---->ConcurrentHashMap<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ConcurrentLinkedQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentLinkedQueue"><!---->ConcurrentLinkedQueue<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/CopyOnWriteArrayList.html" class="nav-link sidebar-link sidebar-page" aria-label="CopyOnWriteArrayList"><!---->CopyOnWriteArrayList<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ThreadLocal.html" class="nav-link sidebar-link sidebar-page" aria-label="ThreadLocal"><!---->ThreadLocal<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/BlockingQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="BlockingQueue"><!---->BlockingQueue<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/pool.html" class="nav-link sidebar-link sidebar-page" aria-label="线程池"><!---->线程池<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ScheduledThreadPoolExecutor.html" class="nav-link sidebar-link sidebar-page" aria-label="计划任务"><!---->计划任务<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/atomic.html" class="nav-link sidebar-link sidebar-page" aria-label="原子操作类"><!---->原子操作类<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/CountDownLatch.html" class="nav-link sidebar-link sidebar-page" aria-label="通信工具类CountDownLatch"><!---->通信工具类CountDownLatch<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/fork-join.html" class="nav-link sidebar-link sidebar-page" aria-label="Fork/Join框架"><!---->Fork/Join框架<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/shengchanzhe-xiaofeizhe.html" class="nav-link sidebar-link sidebar-page" aria-label="生产者-消费者模式"><!---->生产者-消费者模式<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 JVM</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">四、数据库</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">五、计算机基础</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">六、求职面试</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">七、学习资源</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">八、学习建议</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、知识库搭建</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">十、联系作者</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->大致了解下Java的锁接口和锁</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 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id="toc"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#synchronized的不足之处" class="router-link-active router-link-exact-active toc-link level2">synchronized的不足之处</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#锁的几种分类" class="router-link-active router-link-exact-active toc-link level2">锁的几种分类</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#可重入锁和非可重入锁" class="router-link-active router-link-exact-active toc-link level3">可重入锁和非可重入锁</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#公平锁与非公平锁" class="router-link-active router-link-exact-active toc-link level3">公平锁与非公平锁</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#读写锁和排它锁" class="router-link-active router-link-exact-active toc-link level3">读写锁和排它锁</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#jdk中有关锁的一些接口和类" class="router-link-active router-link-exact-active toc-link level2">JDK中有关锁的一些接口和类</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#抽象类aqs-aqls-aos" class="router-link-active router-link-exact-active toc-link level3">抽象类AQS/AQLS/AOS</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#接口condition-lock-readwritelock" class="router-link-active router-link-exact-active toc-link level3">接口Condition/Lock/ReadWriteLock</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#reentrantlock" class="router-link-active router-link-exact-active toc-link level3">ReentrantLock</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#reentrantreadwritelock" class="router-link-active router-link-exact-active toc-link level3">ReentrantReadWriteLock</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/lock.html#stampedlock" class="router-link-active router-link-exact-active toc-link level3">StampedLock</a></li><!----><!--]--></ul><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><h1 id="大致了解下java的锁接口和锁" tabindex="-1"><a class="header-anchor" href="#大致了解下java的锁接口和锁" aria-hidden="true">#</a> 大致了解下Java的锁接口和锁</h1><p>前面我们介绍了Java原生的锁——基于对象的锁，它一般是配合synchronized关键字来使用的。实际上，Java在<code>java.util.concurrent.locks</code>包下，还为我们提供了几个关于锁的类和接口。它们有更强大的功能或更高的性能。</p><h2 id="synchronized的不足之处" tabindex="-1"><a class="header-anchor" href="#synchronized的不足之处" aria-hidden="true">#</a> synchronized的不足之处</h2><p>我们先来看看<code>synchronized</code>有什么不足之处。</p><ul><li>如果临界区是只读操作，其实可以多线程一起执行，但使用synchronized的话，<strong>同一时间只能有一个线程执行</strong>。</li><li>synchronized无法知道线程有没有成功获取到锁</li><li>使用synchronized，如果临界区因为IO或者sleep方法等原因阻塞了，而当前线程又没有释放锁，就会导致<strong>所有线程等待</strong>。</li></ul><p>而这些都是locks包下的锁可以解决的。</p><h2 id="锁的几种分类" tabindex="-1"><a class="header-anchor" href="#锁的几种分类" aria-hidden="true">#</a> 锁的几种分类</h2><p>锁可以根据以下几种方式来进行分类，下面我们逐一介绍。</p><h3 id="可重入锁和非可重入锁" tabindex="-1"><a class="header-anchor" href="#可重入锁和非可重入锁" aria-hidden="true">#</a> 可重入锁和非可重入锁</h3><p>所谓重入锁，顾名思义。就是支持重新进入的锁，也就是说这个锁支持一个<strong>线程对资源重复加锁</strong>。</p><p>synchronized关键字就是使用的重入锁。比如说，你在一个synchronized实例方法里面调用另一个本实例的synchronized实例方法，它可以重新进入这个锁，不会出现任何异常。</p><p>如果我们自己在继承AQS实现同步器的时候，没有考虑到占有锁的线程再次获取锁的场景，可能就会导致线程阻塞，那这个就是一个“非可重入锁”。</p><p><code>ReentrantLock</code>的中文意思就是可重入锁。也是本文后续要介绍的重点类。</p><h3 id="公平锁与非公平锁" tabindex="-1"><a class="header-anchor" href="#公平锁与非公平锁" aria-hidden="true">#</a> 公平锁与非公平锁</h3><p>这里的“公平”，其实通俗意义来说就是“先来后到”，也就是FIFO。如果对一个锁来说，先对锁获取请求的线程一定会先被满足，后对锁获取请求的线程后被满足，那这个锁就是公平的。反之，那就是不公平的。</p><p>一般情况下，<strong>非公平锁能提升一定的效率。但是非公平锁可能会发生线程饥饿（有一些线程长时间得不到锁）的情况</strong>。所以要根据实际的需求来选择非公平锁和公平锁。</p><p>ReentrantLock支持非公平锁和公平锁两种。</p><h3 id="读写锁和排它锁" tabindex="-1"><a class="header-anchor" href="#读写锁和排它锁" aria-hidden="true">#</a> 读写锁和排它锁</h3><p>我们前面讲到的synchronized用的锁和ReentrantLock，其实都是“排它锁”。也就是说，这些锁在同一时刻只允许一个线程进行访问。</p><p>而读写锁可以在同一时刻允许多个读线程访问。Java提供了ReentrantReadWriteLock类作为读写锁的默认实现，内部维护了两个锁：一个读锁，一个写锁。通过分离读锁和写锁，使得在“读多写少”的环境下，大大地提高了性能。</p><blockquote><p>注意，即使用读写锁，在写线程访问时，所有的读线程和其它写线程均被阻塞。</p></blockquote><p><strong>可见，只是synchronized是远远不能满足多样化的业务对锁的要求的</strong>。接下来我们介绍一下JDK中有关锁的一些接口和类。</p><h2 id="jdk中有关锁的一些接口和类" tabindex="-1"><a class="header-anchor" href="#jdk中有关锁的一些接口和类" aria-hidden="true">#</a> JDK中有关锁的一些接口和类</h2><p>众所周知，JDK中关于并发的类大多都在<code>java.util.concurrent</code>（以下简称juc）包下。而juc.locks包看名字就知道，是提供了一些并发锁的工具类的。前面我们介绍的AQS（AbstractQueuedSynchronizer）就是在这个包下。下面分别介绍一下这个包下的类和接口以及它们之间的关系。</p><h3 id="抽象类aqs-aqls-aos" tabindex="-1"><a class="header-anchor" href="#抽象类aqs-aqls-aos" aria-hidden="true">#</a> 抽象类AQS/AQLS/AOS</h3><p>这三个抽象类有一定的关系，所以这里放到一起讲。</p><p>首先我们看<strong>AQS</strong>（AbstractQueuedSynchronizer），之前专门有章节介绍这个类，它是在JDK 1.5 发布的，提供了一个“队列同步器”的基本功能实现。而AQS里面的“资源”是用一个<code>int</code>类型的数据来表示的，有时候我们的业务需求资源的数量超出了<code>int</code>的范围，所以在JDK 1.6 中，多了一个<strong>AQLS</strong>（AbstractQueuedLongSynchronizer）。它的代码跟AQS几乎一样，只是把资源的类型变成了<code>long</code>类型。</p><p>AQS和AQLS都继承了一个类叫<strong>AOS</strong>（AbstractOwnableSynchronizer）。这个类也是在JDK 1.6 中出现的。这个类只有几行简单的代码。从源码类上的注释可以知道，它是用于表示锁与持有者之间的关系（独占模式）。可以看一下它的主要方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 独占模式，锁的持有者 </span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token class-name">Thread</span> exclusiveOwnerThread<span class="token punctuation">;</span>
<span class="token comment">// 设置锁持有者 </span>
<span class="token keyword">protected</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">setExclusiveOwnerThread</span><span class="token punctuation">(</span><span class="token class-name">Thread</span> t<span class="token punctuation">)</span> <span class="token punctuation">{</span>
exclusiveOwnerThread <span class="token operator">=</span> t<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 获取锁的持有线程 </span>
<span class="token keyword">protected</span> <span class="token keyword">final</span> <span class="token class-name">Thread</span> <span class="token function">getExclusiveOwnerThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> exclusiveOwnerThread<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="接口condition-lock-readwritelock" tabindex="-1"><a class="header-anchor" href="#接口condition-lock-readwritelock" aria-hidden="true">#</a> 接口Condition/Lock/ReadWriteLock</h3><p>juc.locks包下共有三个接口：<code>Condition</code>、<code>Lock</code>、<code>ReadWriteLock</code>。其中，Lock和ReadWriteLock从名字就可以看得出来，分别是锁和读写锁的意思。Lock接口里面有一些获取锁和释放锁的方法声明，而ReadWriteLock里面只有两个方法，分别返回“读锁”和“写锁”：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">interface</span> <span class="token class-name">ReadWriteLock</span> <span class="token punctuation">{</span>
<span class="token class-name">Lock</span> <span class="token function">readLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Lock</span> <span class="token function">writeLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>Lock接口中有一个方法是可以获得一个<code>Condition</code>:</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Condition</span> <span class="token function">newCondition</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>之前我们提到了每个对象都可以用继承自<code>Object</code>的<strong>wait/notify</strong>方法来实现<strong>等待/通知机制</strong>。而Condition接口也提供了类似Object监视器的方法，通过与<strong>Lock</strong>配合来实现等待/通知模式。</p><p>那为什么既然有Object的监视器方法了，还要用Condition呢？这里有一个二者简单的对比：</p><table><thead><tr><th>对比项</th><th>Object监视器</th><th>Condition</th></tr></thead><tbody><tr><td>前置条件</td><td>获取对象的锁</td><td>调用Lock.lock获取锁，调用Lock.newCondition获取Condition对象</td></tr><tr><td>调用方式</td><td>直接调用，比如object.notify()</td><td>直接调用，比如condition.await()</td></tr><tr><td>等待队列的个数</td><td>一个</td><td>多个</td></tr><tr><td>当前线程释放锁进入等待状态</td><td>支持</td><td>支持</td></tr><tr><td>当前线程释放锁进入等待状态，在等待状态中不中断</td><td>不支持</td><td>支持</td></tr><tr><td>当前线程释放锁并进入超时等待状态</td><td>支持</td><td>支持</td></tr><tr><td>当前线程释放锁并进入等待状态直到将来的某个时间</td><td>不支持</td><td>支持</td></tr><tr><td>唤醒等待队列中的一个线程</td><td>支持</td><td>支持</td></tr><tr><td>唤醒等待队列中的全部线程</td><td>支持</td><td>支持</td></tr></tbody></table><p>Condition和Object的wait/notify基本相似。其中，Condition的await方法对应的是Object的wait方法，而Condition的<strong>signal/signalAll</strong>方法则对应Object的notify/notifyAll()。但Condition类似于Object的等待/通知机制的加强版。我们来看看主要的方法：</p><table><thead><tr><th>方法名称</th><th>描述</th></tr></thead><tbody><tr><td>await()</td><td>当前线程进入等待状态直到被通知（signal）或者中断；当前线程进入运行状态并从await()方法返回的场景包括：（1）其他线程调用相同Condition对象的signal/signalAll方法，并且当前线程被唤醒；（2）其他线程调用interrupt方法中断当前线程；</td></tr><tr><td>awaitUninterruptibly()</td><td>当前线程进入等待状态直到被通知，在此过程中对中断信号不敏感，不支持中断当前线程</td></tr><tr><td>awaitNanos(long)</td><td>当前线程进入等待状态，直到被通知、中断或者超时。如果返回值小于等于0，可以认定就是超时了</td></tr><tr><td>awaitUntil(Date)</td><td>当前线程进入等待状态，直到被通知、中断或者超时。如果没到指定时间被通知，则返回true，否则返回false</td></tr><tr><td>signal()</td><td>唤醒一个等待在Condition上的线程，被唤醒的线程在方法返回前必须获得与Condition对象关联的锁</td></tr><tr><td>signalAll()</td><td>唤醒所有等待在Condition上的线程，能够从await()等方法返回的线程必须先获得与Condition对象关联的锁</td></tr></tbody></table><h3 id="reentrantlock" tabindex="-1"><a class="header-anchor" href="#reentrantlock" aria-hidden="true">#</a> ReentrantLock</h3><p>ReentrantLock是一个非抽象类，它是Lock接口的JDK默认实现，实现了锁的基本功能。从名字上看，它是一个”可重入“锁，从源码上看，它内部有一个抽象类<code>Sync</code>，是继承了AQS，自己实现的一个同步器。同时，ReentrantLock内部有两个非抽象类<code>NonfairSync</code>和<code>FairSync</code>，它们都继承了Sync。从名字上看得出，分别是”非公平同步器“和”公平同步器“的意思。这意味着ReentrantLock可以支持”公平锁“和”非公平锁“。</p><p>通过看这两个同步器的源码可以发现，它们的实现都是”独占“的。都调用了AOS的<code>setExclusiveOwnerThread</code>方法，所以ReentrantLock的锁是”独占“的，也就是说，它的锁都是”排他锁“，不能共享。</p><p>在ReentrantLock的构造方法里，可以传入一个<code>boolean</code>类型的参数，来指定它是否是一个公平锁，默认情况下是非公平的。这个参数一旦实例化后就不能修改，只能通过<code>isFair()</code>方法来查看。</p><h3 id="reentrantreadwritelock" tabindex="-1"><a class="header-anchor" href="#reentrantreadwritelock" aria-hidden="true">#</a> ReentrantReadWriteLock</h3><p>这个类也是一个非抽象类，它是ReadWriteLock接口的JDK默认实现。它与ReentrantLock的功能类似，同样是可重入的，支持非公平锁和公平锁。不同的是，它还支持”读写锁“。</p><p>ReentrantReadWriteLock内部的结构大概是这样：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 内部结构</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">ReentrantReadWriteLock<span class="token punctuation">.</span>ReadLock</span> readerLock<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">ReentrantReadWriteLock<span class="token punctuation">.</span>WriteLock</span> writerLock<span class="token punctuation">;</span>
<span class="token keyword">final</span> <span class="token class-name">Sync</span> sync<span class="token punctuation">;</span>
<span class="token keyword">abstract</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">Sync</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractQueuedSynchronizer</span> <span class="token punctuation">{</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">NonfairSync</span> <span class="token keyword">extends</span> <span class="token class-name">Sync</span> <span class="token punctuation">{</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">FairSync</span> <span class="token keyword">extends</span> <span class="token class-name">Sync</span> <span class="token punctuation">{</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">ReadLock</span> <span class="token keyword">implements</span> <span class="token class-name">Lock</span><span class="token punctuation">,</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">Sync</span> sync<span class="token punctuation">;</span>
<span class="token keyword">protected</span> <span class="token class-name">ReadLock</span><span class="token punctuation">(</span><span class="token class-name">ReentrantReadWriteLock</span> lock<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sync <span class="token operator">=</span> lock<span class="token punctuation">.</span>sync<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">WriteLock</span> <span class="token keyword">implements</span> <span class="token class-name">Lock</span><span class="token punctuation">,</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">Sync</span> sync<span class="token punctuation">;</span>
<span class="token keyword">protected</span> <span class="token class-name">WriteLock</span><span class="token punctuation">(</span><span class="token class-name">ReentrantReadWriteLock</span> lock<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sync <span class="token operator">=</span> lock<span class="token punctuation">.</span>sync<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
<span class="token comment">// 构造方法，初始化两个锁</span>
<span class="token keyword">public</span> <span class="token class-name">ReentrantReadWriteLock</span><span class="token punctuation">(</span><span class="token keyword">boolean</span> fair<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sync <span class="token operator">=</span> fair <span class="token operator">?</span> <span class="token keyword">new</span> <span class="token class-name">FairSync</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">NonfairSync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
readerLock <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ReadLock</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
writerLock <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">WriteLock</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 获取读锁和写锁的方法</span>
<span class="token keyword">public</span> <span class="token class-name">ReentrantReadWriteLock<span class="token punctuation">.</span>WriteLock</span> <span class="token function">writeLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> writerLock<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">ReentrantReadWriteLock<span class="token punctuation">.</span>ReadLock</span> <span class="token function">readLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> readerLock<span class="token punctuation">;</span> <span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>可以看到，它同样是内部维护了两个同步器。且维护了两个Lock的实现类ReadLock和WriteLock。从源码可以发现，这两个内部类用的是外部类的同步器。</p><p>ReentrantReadWriteLock实现了读写锁，但它有一个小弊端，就是在“写”操作的时候，其它线程不能写也不能读。我们称这种现象为“写饥饿”，将在后文的StampedLock类继续讨论这个问题。</p><h3 id="stampedlock" tabindex="-1"><a class="header-anchor" href="#stampedlock" aria-hidden="true">#</a> StampedLock</h3><p><code>StampedLock</code>类是在Java 8 才发布的，也是Doug Lea大神所写，有人号称它为锁的性能之王。它没有实现Lock接口和ReadWriteLock接口，但它其实是实现了“读写锁”的功能，并且性能比ReentrantReadWriteLock更高。StampedLock还把读锁分为了“乐观读锁”和“悲观读锁”两种。</p><p>前面提到了ReentrantReadWriteLock会发生“写饥饿”的现象，但StampedLock不会。它是怎么做到的呢？它的核心思想在于，<strong>在读的时候如果发生了写，应该通过重试的方式来获取新的值，而不应该阻塞写操作。这种模式也就是典型的无锁编程思想，和CAS自旋的思想一样</strong>。这种操作方式决定了StampedLock在读线程非常多而写线程非常少的场景下非常适用，同时还避免了写饥饿情况的发生。</p><p>这里篇幅有限，就不介绍StampedLock的源码了，只是分析一下官方提供的用法（在JDK源码类声明的上方或Javadoc里可以找到）。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Point</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">double</span> x<span class="token punctuation">,</span> y<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">StampedLock</span> sl <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StampedLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 写锁的使用</span>
<span class="token keyword">void</span> <span class="token function">move</span><span class="token punctuation">(</span><span class="token keyword">double</span> deltaX<span class="token punctuation">,</span> <span class="token keyword">double</span> deltaY<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> stamp <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">writeLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取写锁</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
x <span class="token operator">+=</span> deltaX<span class="token punctuation">;</span>
y <span class="token operator">+=</span> deltaY<span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
sl<span class="token punctuation">.</span><span class="token function">unlockWrite</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 释放写锁</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// 乐观读锁的使用</span>
<span class="token keyword">double</span> <span class="token function">distanceFromOrigin</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> stamp <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">tryOptimisticRead</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取乐观读锁</span>
<span class="token keyword">double</span> currentX <span class="token operator">=</span> x<span class="token punctuation">,</span> currentY <span class="token operator">=</span> y<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>sl<span class="token punctuation">.</span><span class="token function">validate</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// //检查乐观读锁后是否有其他写锁发生，有则返回false</span>
stamp <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">readLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取一个悲观读锁</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
currentX <span class="token operator">=</span> x<span class="token punctuation">;</span>
currentY <span class="token operator">=</span> y<span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
sl<span class="token punctuation">.</span><span class="token function">unlockRead</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 释放悲观读锁</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token class-name">Math</span><span class="token punctuation">.</span><span class="token function">sqrt</span><span class="token punctuation">(</span>currentX <span class="token operator">*</span> currentX <span class="token operator">+</span> currentY <span class="token operator">*</span> currentY<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 悲观读锁以及读锁升级写锁的使用</span>
<span class="token keyword">void</span> <span class="token function">moveIfAtOrigin</span><span class="token punctuation">(</span><span class="token keyword">double</span> newX<span class="token punctuation">,</span> <span class="token keyword">double</span> newY<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> stamp <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">readLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 悲观读锁</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span>x <span class="token operator">==</span> <span class="token number">0.0</span> <span class="token operator">&&</span> y <span class="token operator">==</span> <span class="token number">0.0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 读锁尝试转换为写锁：转换成功后相当于获取了写锁，转换失败相当于有写锁被占用</span>
<span class="token keyword">long</span> ws <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">tryConvertToWriteLock</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ws <span class="token operator">!=</span> <span class="token number">0L</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 如果转换成功</span>
stamp <span class="token operator">=</span> ws<span class="token punctuation">;</span> <span class="token comment">// 读锁的票据更新为写锁的</span>
x <span class="token operator">=</span> newX<span class="token punctuation">;</span>
y <span class="token operator">=</span> newY<span class="token punctuation">;</span>
<span class="token keyword">break</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token comment">// 如果转换失败</span>
sl<span class="token punctuation">.</span><span class="token function">unlockRead</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 释放读锁</span>
stamp <span class="token operator">=</span> sl<span class="token punctuation">.</span><span class="token function">writeLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 强制获取写锁</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
sl<span class="token punctuation">.</span><span class="token function">unlock</span><span class="token punctuation">(</span>stamp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 释放所有锁</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><blockquote><p>乐观读锁的意思就是先假定在这个锁获取期间，共享变量不会被改变，既然假定不会被改变，那就不需要上锁。在获取乐观读锁之后进行了一些操作，然后又调用了validate方法，这个方法就是用来验证tryOptimisticRead之后，是否有写操作执行过，如果有，则获取一个悲观读锁，这里的悲观读锁和ReentrantReadWriteLock中的读锁类似，也是个共享锁。</p></blockquote><p>可以看到，StampedLock获取锁会返回一个<code>long</code>类型的变量，释放锁的时候再把这个变量传进去。简单看看源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 用于操作state后获取stamp的值</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> LG_READERS <span class="token operator">=</span> <span class="token number">7</span><span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> RUNIT <span class="token operator">=</span> <span class="token number">1L</span><span class="token punctuation">;</span> <span class="token comment">//0000 0000 0001</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> WBIT <span class="token operator">=</span> <span class="token number">1L</span> <span class="token operator"><<</span> LG_READERS<span class="token punctuation">;</span> <span class="token comment">//0000 1000 0000</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> RBITS <span class="token operator">=</span> WBIT <span class="token operator">-</span> <span class="token number">1L</span><span class="token punctuation">;</span> <span class="token comment">//0000 0111 1111</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> RFULL <span class="token operator">=</span> RBITS <span class="token operator">-</span> <span class="token number">1L</span><span class="token punctuation">;</span> <span class="token comment">//0000 0111 1110</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> ABITS <span class="token operator">=</span> RBITS <span class="token operator">|</span> WBIT<span class="token punctuation">;</span> <span class="token comment">//0000 1111 1111</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> SBITS <span class="token operator">=</span> <span class="token operator">~</span>RBITS<span class="token punctuation">;</span> <span class="token comment">//1111 1000 0000</span>
<span class="token comment">// 初始化时state的值</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> ORIGIN <span class="token operator">=</span> WBIT <span class="token operator"><<</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">//0001 0000 0000</span>
<span class="token comment">// 锁共享变量state</span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token keyword">volatile</span> <span class="token keyword">long</span> state<span class="token punctuation">;</span>
<span class="token comment">// 读锁溢出时用来存储多出的读锁</span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token keyword">int</span> readerOverflow<span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>StampedLock用这个long类型的变量的前7位（LG_READERS）来表示读锁，每获取一个悲观读锁，就加1（RUNIT），每释放一个悲观读锁，就减1。而悲观读锁最多只能装128个（7位限制），很容易溢出，所以用一个int类型的变量来存储溢出的悲观读锁。</p><p>写锁用state变量剩下的位来表示，每次获取一个写锁，就加0000 1000 0000（WBIT）。需要注意的是，<strong>写锁在释放的时候，并不是减WBIT，而是再加WBIT</strong>。这是为了<strong>让每次写锁都留下痕迹</strong>，解决CAS中的ABA问题，也为<strong>乐观锁检查变化</strong>validate方法提供基础。</p><p>乐观读锁就比较简单了，并没有真正改变state的值，而是在获取锁的时候记录state的写状态，在操作完成后去检查state的写状态部分是否发生变化，上文提到了，每次写锁都会留下痕迹，也是为了这里乐观锁检查变化提供方便。</p><p>总的来说，StampedLock的性能是非常优异的，基本上可以取代ReentrantReadWriteLock的作用。</p><hr><blockquote><p>编辑：沉默王二，内容大部分来源以下三个开源仓库：</p><ul><li><a href="http://concurrent.redspider.group/" target="_blank" rel="noopener noreferrer">深入浅出 Java 多线程<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span 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